Traditional solutions for providing increased RF coverage and/or call capacity in wireless communications systems rely on some means of distributing RF energy. These range from high power, large coverage area towers to low power in-building pico-cells. There also exists a class of RF enhancement technologies known as RF repeaters. Some are bidirectional RF amplifiers that retransmit the signals received over the air from a host base station, known as over the air repeaters. Others are directly connected to a host base station and distribute the RF signals via either electrical (e.g. coaxial cable) or optical (fiber) distribution networks, known as distributed antenna systems. In some cases, over the air repeaters are combined with these fiber or coaxial based distributed antenna systems.
In all cases over the air repeater systems have significant uplink performance limits when finite power amplifier output capability is combined with input signals simultaneously spanning weak and strong signal levels. Over the air repeater systems encounter further performance limits, for example, the Global System for Mobile Communication (GSM) signals utilize the frequency hopping mode of operation which in conjunction with dynamic range limits severely limits the choice in repeater design and output capability.
More specifically, over the air repeaters bi-directionally amplify wireless signals received from donor base stations on the downlink and those received from the mobile users on the uplink. The gain is usually set as high as possible to compensate for the path loss between the repeater and the donor base station while staying low enough to prevent the repeater's output noise from causing a significant noise rise at the donor base station.
GSM repeaters, especially multi-carrier sub-band units, are confronted with the added challenge of the wide dynamic range differences due to concurrently amplifying signals from both near and far mobile units. Many repeaters have been constructed that use separate RF chains and power amplifiers for each RF channel. This allows separate processing, gain settings and power amplifier output limits for each signal. However, this approach is not cost effective to support a moderate or large number of simultaneous RF channels and may result in impractically sized product solutions. Large carrier counts need broadband repeater solutions which use multi-carrier power amplifiers (MCPA). However, the use of MCPAs on the uplink limits the useful gain range of these types of repeaters and tend to be limited to lower power applications.
The simplest form of a MCPA based repeater will use a multicarrier RF chain in conjunction with the power amplifier (PA). This means that any gain reduction to prevent PA overload due to a strong input signal will also result in the gain reduction of any weak signal(s) being simultaneously received. Thus, the weak signals will not be able to communicate with the donor base station, which is equivalent to reducing the reverse link coverage range of the repeater. Some repeaters may utilize digital processing of the RF signals but will still treat groups of RF channels as a single entity with the associated gain coupling limitations.
As is well known in the art, digital filtering has been used to isolate individual RF channels, to eliminate interfering signals from other wireless operators bands or those intended for other cell sites and not the donor base station of interest. However, limiting the output level of the MCPA through gain changes still couples the effects due to strong signals to the weak signals. This still does not address the limitations imposed by a shared MCPA at the output.
It is possible to implement dynamic gain control over each sub-band/RF channel individually during the digital filtering process. Gain limits can be applied digitally that prevent strong signals from either overloading the power amplifier and can be dynamic enough to adapt to burst level differences in a GSM waveform. However, it takes a finite amount of time to detect the presence of a strong signal and during that time period the PA output will be overloaded. Conversely, starting at low gain will require some time before a weak signal detection decision can be used to increase gain. This is further complicated by the presence of frequency hopping which adds frequency uncertainty to the existing power versus time uncertainty. Finally, none of the prior art addresses techniques to accommodate feedback cancellation operations and the presence of strong feedback signals at the input.